Alkyl halides are useful solvents because of the polarity it introduced to the hydrocarbon. It is used as solvent for industrial manufacturing processes and the cleaning industry. However because of the polarity it is also harmful to the biological system. Chloroform was at one time used as an anesthetic. It is now considered a carcinogen and the practice has been discontinued. One of the most popular pesticides in the 1930s is 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane otherwise known as DDT.

The parent compound is ethane. The carbon with the chlorine is designated as carbon-1.   The carbon of the benzene ring attached to the ethane is designated as carbon-1 and the chlorine is attached to the para-position with respect to this carbon-1. Bis means two of such units.

DDT is such an effective general pesticide that it was widely used, resulting in high amount being present in the soil and finally in the water. Also it breaks down very slowly, so the lower organisms of the food chain - plankton - contain a high concentration of this pesticide. Eventually it moved up through the food chain - fish and animals - and end up in humans. It was shown to be harmful and in 1972 it was banned in the United States of America.

Other chlorine-containing compounds like carbon tetrachloride and 1,1,1-trichloroethane are widely used as solvents and industrial chemicals. Methyl bromide, which can release its bromine, became an effective soil fumigant. Carbon tetrachloride is now labelled a carcinogen and its use has been greatly restricted.

The early refrigerant used was ammonia since its boiling point is −33.5�C, not too below room temperature. Ammonia is of course corrosive and toxic. Alkyl halides were good substitutes as they are stable to heat and are not corrosive. By having the correct amount of fluorine and chlorine in methane and ethane it is easy to prepare liquids with boiling points just below the temperature of the room.

For over 50 years, chlorofluorocarbons (CFCs) were thought of as miracle substances. They are stable, nonflammable, low in toxicity, and inexpensive to produce. Over time, CFCs found uses as refrigerants, solvents, foam blowing agents, and in other smaller applications. So a series of freons were prepared.

Freons are the commercial names for chlorofluoroalkanes. Freon 10 means 1 carbon and no fluorine (by inference the number of chlorine is four). Chlorofluoroalkanes are commercially known as chlorofluorocarbons, or CFCs.

All of these compounds have atmospheric lifetimes long enough to allow them to be transported by winds into the stratosphere. Because they release chlorine or bromine when they break down, they damage the protective ozone layer.

CHLOROFLUOROCARBONS (CFCs)

The Earth's atmosphere is divided into several layers. The lowest region, the troposphere, extends from the Earth's surface up to about 10 km. Mt. Everest, the tallest mountain on the planet, is about 9 km high. The next layer, the stratosphere, is from 10 km to about 50 km. Most commercial airlines in the lower region of the stratosphere.

Atmospheric ozone is found mainly about 15-30 kilometers above the Earth's surface. The concentration is about 3 moles of ozone for every 2 million moles of oxygen gas. (20% of air molecules are oxygen). This ozone layer has the important role of filtering out Ultraviolet (UV) radiation from the sun; preventing it from reaching the earth's surface.

UV radiations are electromagnetic radiation with wavelengths from 280 nanometers (nm) to 400 nm. For its reaction with ozone and oxygen it is useful to classify the radiation into UVA (from 320-400 nm), UVB (from 280-320 nm), and UVC (below 280 nm). UVA is not absorbed by ozone. UVB is partially absorbed by ozone, while UVC is completely absorbed by ozone and oxygen gas. UVB can cause cataract, melanoma and other types of skin cancer, and has been linked to damage of crops, and marine organisms. UVC is extremely dangerous.

Ozone concentrations varies with seasons, especially with sunspots. However records spanning several decades showed that its concentration in the stratosphere remains relatively constant, up till the later part of the 20th century, when evidence showed that the ozone shield is being depleted well beyond changes due to natural processes.

In the early 1970s, researchers began to investigate the effects of various chemicals on the ozone layer, particularly the halogenated hydrocarbons. The alkylchlorides are stable in nature and are not dissolve by rain. Over time, winds drive them into the stratosphere where they are broken up by strong UV radiation to release atomic chlorine by a free radical reaction. One chlorine atom released can destroy over 100,000 ozone molecules in a chain mechanism. Numerous experiments have shown that chlorofluorocarbons might be responsible for up to 85% of the chlorine in the stratosphere, while natural sources contribute only 15%. Chlorine produced in the troposphere - from swimming pools, industrial plants, sea salt, and volcanoes - readily dissolves in water and rain, and removed from the troposphere very quickly.

A sign of the ozone depletion in the stratosphere is the annual (during the Antarctic Spring) ozone "hole" over Antarctica since the early 1980s. In this hole, ozone concentration fall by over 60% during the worst years.

Environmentalists has defined Ozone Depletion Potential (ODP) of a compound as the ratio of the impact the compound has on ozone as compared to a similar mass of CFC-11 (i.e. the ODP of CFC-11 is defined to be 1.0).

Halon, a class of compounds consisting of bromine, fluorine, and carbon, are used as extinguishing agents in fire extinguishers. Technically, all compounds containing carbon and fluorine and/or chlorine are halons, but the U.S. the Clean Air Act defined "halon" as a fire-extinguishing agent. Bromine is many times more effective at destroying ozone than chlorine. Halon 1301 (ODP = 10) and Halon 1211 (ODP = 3).

The Montreal Protocol of 1987, supported by about 140 countries, had worked to stop production of halons by the beginning of 1994 and of CFCs by the beginning of 1996 in developed countries.

The general understanding is that the bromo and the chloro are considered unacceptable. In phasing out the CFCs many are temporarily using HCFCs (hydrochlorofluorocarbon) in the understanding that there are less chlorine per molecule. The goal is to progress to HFCs (hydrofluorocarbon) said to show ODP = 0.